This invention relates to apparatus for acquiring data used to evaluate and reproduce the color of a sample on the basis of the chroma and glossiness of the sample.
The inventor of the present invention has carried out studies on the automatic decision of colors, on the detection of unevenness in colors of painted materials, and on the permissible range of unevenness.
Based on these studies, the inventor found that the grade of unevenness in colors which can be perceived by the eyes of a normal human being depends on the race to which the human being belongs (or on the color of the eyes). The normal human being described here implies an adult with no ophthalmic disease, who belongs to a group of adults with the same eye color, excepting those who have received special training. In relation to this knowledge, the inventor of the present invention published a paper "Automatic color Difference Discrimination System with Capability of the Dark Eyes' Color Perception," appearing in the Record of the 1EEE Conference on TENCON '91, New Delhi, India, vol. 3, pp. 401-404, August 1991. In addition, the automatic color difference discrimination system which was developed in accordance with the principle of operation of that paper was disclosed as Japanese patent disclosure number 232821 in 1992.
The color difference threshold, as appearing in the patent disclosure with Japanese patent disclosure number 232821 in 1992, was measured in accordance with the color difference by the method of constant stimuli in order to find the range of the color difference threshold. The standard stimuli for obtaining the range of the color difference threshold was generated by passing a light beam through interference filters with center frequencies of 430 nm (purple), 480 nm (blue), 535 nm (green), 580 nm (yellow), 600 nm (orange), and 655 nm (red).
FIG. 1 shows the average of the measured values of the color difference threshold at specific light wavelength, and the 95% confidence interval of these value.
The lower and upper boundaries to the subjective equivalent color in terms of the wavelength were thus obtained from the plots of FIG. 1 when the standard stimuli was given.
The interval between the lower and upper boundaries to the subjective equivalent color in terms of the specific wavelength gradually decreases with the wavelength increasing from purple to yellow, and reaches the minimum at the wavelength of orange. This interval then increases with the increasing wavelength starting from orange until arriving at the wavelength of red.
The interpolating method applies to the lower and upper boundaries to the subjective equivalent color, which are given in terms of the wavelength, so that a set of regression curves with minimum standard deviations could be obtained. Expressions (1) and (2) represent the regression curves for the upper (Y.sub.1) and lower (Y.sub.2) boundaries, respectively.
For the upper boundary (Y.sub.1) to the subjective equivalent color, expression (1) is given as EQU Y.sub.1 =2.18.times.10.sup.-6 X.sup.3 -2.36.times.10.sup.-3 X.sup.2 +1.63X(1)
For the lower boundary (Y.sub.2) to the subjective equivalent color, expression (2) is EQU Y.sub.2 =1.95.times.10.sup.-4 X.sup.2 +0.88X (2)
Both Y.sub.1 and Y.sub.2 are valid for the wavelengths in the range of 410 nm to 670 nm.
The interval in wavelength, which is defined by a set of expressions (1) and (2), indicates the threshold to discriminate the subjective equivalent color in terms of the specific wavelength.